1. Field of the Invention
The invention relates generally to ground fault circuit interrupters and, more particularly, to ground fault circuit interrupters capable of detecting low levels of ground fault current through a human being in the presence of large standing leakages currents.
2. Description of the Prior Art
Circuit interrupters are widely used in industrial, commercial, and residential environments to protect circuit conductors against damage due to overcurrent conditions. Such circuit interrupters are normally designed to open, or trip, whenever current flow through the interrupter reaches a level which may cause damage to circuit components due to resistance heating of the conductors. However, damage can also occur at levels significantly lower than the overcurrent trip level if a path is established from a line conductor to ground. Therefore, ground fault circuit interrupters are provided to detect such ground fault current flow and trip at levels significantly below the overcurrent trip level.
Even greater sensitivity is required in order to provide protection to human beings who accidently establish a fault current path from a live conductor to ground. The value of 5 milliamperes has been generally accepted as the desirable level for ground fault circuit interrupters to trip in order to provide people protection. A ground fault circuit interrupter having a trip level of 5 milliamperes is generally satisfactory in a residential environment or in other applications having a low standing leakage current. However, many industrial installations which employ large amounts of electrical machinery have leakage currents much greater than 5 milliamperes. A standard people-protecting ground fault circuit interrupter is thus not applicable under such conditions. It would be desirable to provide a ground fault circuit interrupter capable of withstanding large standing leakage currents caused by ground fault paths through electrical apparatus but which will trip upon establishment of a ground fault path through a human being.